normal distribution

im having problem with this question:

The mass of a grade A papaya follows a normal distribution with mean 1.8 kg and standard deviation 200 g. The mass of a grade B papaya follows a normal distribution with mean 1.5 kg and standard deviation 140 g. Determine the probability that
a)3 grade A papayas chosen at random has a total mass of less than 5 kg,

b)3 times the mass of a grade A papaya chosen at random is less than 5 kg,

c)the total mass of 6 grade A papayas is more than that of 7 grade B papayas, all chosen at random.